Extra ribs may have been a harbinger of doom for the wooly rhino

A new study has found that one in three members of the extinct species had an extra rib that often indicates other congenital abnormalities, writes Tim Wallace.

An artist’s impression of wooly rhinos is a frosty Pleistocene winter.

Corey Ford / Stocktrek Images

A cervical rib, for the one in 200 humans born with the congenital condition, is likely to be at least an occasional pain in the neck. While an extra rib-bone growing from the base of the neck just above the collarbone is, in itself, considered relatively harmless, its presence is strongly associated with other major congenital abnormalities. Just 1 in 10 of those with the condition survive to adulthood.

For the woolly rhinoceros (Coelodonta antiquitatis), which roamed the northern climes of Europe and Asia for hundreds of thousands years until as recently as 10,000 years ago, the condition was more than just a pain in the neck, indicating a species in terminal decline, spiraling towards extinction.

The skeleton of a wooly rhinoceros.

Robyn Beck / Getty

Analysing 32 specimens of the extinct megaherbivore dredged from the North Sea and river deltas in the Netherlands, researchers Alexandra van der Geer and Frietson Galis, of the Naturalis Biodiversity Center in Leiden, have found that fully one-third had cervical ribs – an unusually high incidence compared to extant rhinos and elephants.

The finding, published in PeerJ, echoes previous research in which Galis was involved, that found a remarkably high percentage of cervical ribs among woolly mammoth.

The “exceptionally high incidence” of large cervical ribs in Late Pleistocene woolly rhinos and mammoths, der Geer and Galis suggest, could be due to two factors.

First, there is the possibility of a high rate of inbreeding in declining populations, “potentially resulting in drift and preservation of less favorable genetic variants”. Second, the high incidence of cervical ribs might have been due to harsh climatic conditions affecting pregnancies, “because diseases, famine, cold and other stressors can lead to disturbances of early development”.

Changes in habitat, particularly increased snowfall, might have placed pressure on the sources of sustenance favoured by wooly mammoths and rhinos. As herbs and mosses lost ground with the spread of less nutritious shrubs and grasses, mineral deficiencies suffered by the megaherbivores translated into degenerative bone conditions.

Der Geer and Galis conclude the high incidence and large size of the cervical ribs in woolly mammoths and rhinoceros indicates a strong vulnerability, associated with diseases and congenital abnormalities in mammals in general, that may well have contributed to their eventual extinction.